I am assuming that the police have some pretty hard evidence to support a First degree murder charge, but as a person who lives daily with someone who has severe combat related PTSD, and a member of the SCA who has had interactions with both Melissa and Howard, I still do not believe that this was a premeditated murder. Innocent people have been convicted in the past on circumstantial evidence, and it is well within the realm of possibility that something happened between Howard and Melissa that Howard isn't telling people which led to her leaving the house, etc.
Tell me Howard had a PTSD episode, became disassociated and killed her, and I might believe it. However, without hard, physical evidence I can not believe that the Warrior Poet I knew would deliberately kill a woman he was so completely in love with, not even for money! Besides, I believe if it was a deliberate kill for financial gain, a soldier of Howard's experience and expertise would have found a much better way to make it look like an accident than this.
As to the separate cars, that is easy. Melissa would probably drive from work to whatever SCA gathering they were attending on whichever night, and Howard would drive there from a different place, either work or home. They would meet up there, and then drive their vehicles to either home, or out for dinner with the group. A lot of SCA couples attend meetings that way.